Notifying Objects That Are Associated with a Session

Recall that your application can notify web context
and session listener objects of servlet life-cycle events (Handling Servlet Life-Cycle Events). You can also
notify objects of certain events related to their association with a session
such as the following:

When the session to which the object is attached will be passivated
or activated. A session will be passivated or activated when it is moved between
virtual machines or saved to and restored from persistent storage. To receive
this notification, your object must implement the javax.servlet.http.HttpSessionActivationListener interface.